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hormone,secretory substance carried from one gland or organ of the body via the bloodstream to more or less specific tissues, where it exerts some influence upon the metabolism of the target tissue. Normally, various hormones are produced and secreted by the endocrine glands (see endocrine systemendocrine system
, body control system composed of a group of glands that maintain a stable internal environment by producing chemical regulatory substances called hormones.
..... Click the link for more information. ), including the pituitary, thyroid, parathyroids, adrenals, ovaries, testes, pancreatic islets, certain portions of the gastrointestinal tract, and the placenta, among the mammalian species. As lack of any one of them may cause serious disorders, many hormones are now produced synthetically and used in treatment where a deficiency exists. The hormones of the anterior pituitary include thyrotropinthyrotropin
or thyroid-stimulating hormone
(TSH), hormone released by the anterior pituitary gland that stimulates the thyroid gland to release thyroxine. The release of thyrotropin is triggered by the action of thyrotropin-releasing factor (TRF), a substance found in
..... Click the link for more information. , adrenocorticotropic hormoneadrenocorticotropic hormone
, polypeptide hormone secreted by the anterior pituitary gland. Its chief function is to stimulate the cortex of the adrenal gland to secrete adrenocortical steroids, chief among them cortisone.
..... Click the link for more information. , the gonadotropic hormonesgonadotropic hormone
any one of three glycoprotein (see protein) hormones released by either the anterior pituitary gland or the placenta (the organ in which maternal and fetal blood exchange nutrients and waste products) that have various effects upon
..... Click the link for more information. , and growth hormonegrowth hormone
, glycoprotein hormone released by the anterior pituitary gland that is necessary for normal skeletal growth in humans (see protein).
..... Click the link for more information. ; the posterior pituitary secretes antidiuretic hormoneantidiuretic hormone
, polypeptide hormone secreted by the posterior pituitary gland. Its principal action is to regulate the amount of water excreted by the kidneys. Antidiuretic hormone (ADH), known also as vasopressin, causes the kidneys to resorb water directly from the
..... Click the link for more information. , prolactin, and oxytocinoxytocin
, hormone released from the posterior lobe of the pituitary gland that facilitates uterine contractions and the milk-ejection reflex. The structure of oxytocin, a cyclic peptide consisting of nine amino acids, was determined in 1953 and in the same year it was
..... Click the link for more information. . The thyroids secrete thyroxinethyroxine
, substance secreted by the thyroid gland. The hormone thyroxine forms by combining the amino acid tyrosine with iodine. Complexed to a protein, it is stored in the follicle stems between thyroid cells.
..... Click the link for more information. and calcitonin, and the parathyroids secrete parathyroid hormoneparathyroid hormone
a hormone secreted by the parathyroid glands that regulates the metabolism of calcium and phosphate in the body. It has been purified extensively and appears to be a protein containing 84 amino-acid residues, a sequence of which
..... Click the link for more information. . The adrenal medulla secretes epinephrineepinephrine
, hormone important to the body's metabolism, also known as adrenaline. Epinephrine, a catecholamine, together with norepinephrine, is secreted principally by the medulla of the adrenal gland.
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, a neurotransmitter in the catecholamine family that mediates chemical communication in the sympathetic nervous system, a branch of the autonomic nervous system.
..... Click the link for more information. while the cortex of the same gland releases aldosteronealdosterone
, steroid secreted by the cortex of the adrenal gland. It is the most potent hormone regulating the body's electrolyte balance. Aldosterone acts directly on the kidney to decrease the rate of sodium-ion excretion (with accompanying retention of water), and to
..... Click the link for more information. , corticosteronecorticosterone
, steroid hormone secreted by the outer layer, or cortex, of the adrenal gland. Classed as a glucocorticoid, corticosterone helps regulate the conversion of amino acids into carbohydrates and glycogen by the liver, and helps stimulate glycogen formation in the
..... Click the link for more information. , cortisolcortisol
steroid hormone that in humans is the major circulating hormone of the cortex, or outer layer, of the adrenal gland. Like cortisone, cortisol is classed as a glucocorticoid; it stimulates liver glycogen formation while it decreases the rate
..... Click the link for more information. , and cortisonecortisone
, steroid hormone whose main physiological effect is on carbohydrate metabolism. It is synthesized from cholesterol in the outer layer, or cortex, of the adrenal gland under the stimulation of adrenocorticotropic hormone (ACTH).
..... Click the link for more information. . The ovaries primarily secrete estrogenestrogen
, any one of a group of hormones synthesized by the reproductive organs and adrenal glands in females and, in lesser quantities, in males. The estrogens cause the thickening of the lining of the uterus and vagina in the early phase of the ovulatory, or menstrual, cycle
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, female sex hormone that induces secretory changes in the lining of the uterus essential for successful implantation of a fertilized egg. A steroid, progesterone is secreted chiefly by the corpus luteum, a group of cells formed in the ovary after the follicle
..... Click the link for more information. and the testes testosteronetestosterone
, principal androgen, or male sex hormone. One of the group of compounds known as anabolic steroids, testosterone is secreted by the testes (see testis) but is also synthesized in small quantities in the ovaries, cortices of the adrenal glands, and placenta, usually
..... Click the link for more information. . The adrenal cortex, ovaries, and testes in fact produce at least small amounts of all of the steroidsteroids,
class of lipids having a particular molecular ring structure called the cyclopentanoperhydro-phenanthrene ring system. Steroids differ from one another in the structure of various side chains and additional rings. Steroids are common in both plants and animals.
..... Click the link for more information. hormones. The islets of Langerhans in the pancreas secrete insulininsulin,
hormone secreted by the β cells of the islets of Langerhans, specific groups of cells in the pancreas. Insufficiency of insulin in the body results in diabetes. Insulin was one of the first products to be manufactured using genetic engineering.
..... Click the link for more information. , glucagonglucagon
, hormone secreted by the α cells of the islets of Langerhans, specific groups of cells in the pancreas. It tends to counteract the action of insulin, i.e., it raises the concentration of glucose in the blood.
..... Click the link for more information. , and somatostatin. The kidneys also produce erythropoietin, which produces erythrocytes (red blood cells). The passage of chyme (see digestive systemdigestive system,
in the animal kingdom, a group of organs functioning in digestion and assimilation of food and elimination of wastes. Virtually all animals have a digestive system. In the vertebrates (phylum Chordata, subphylum Vertebrata) the digestive system is very complex.
..... Click the link for more information. ) from the stomach to the duodenum causes the latter to release secretin, which stimulates the flow of pancreatic juice. The duodenum can also be stimulated by the presence of fats in the chyme to secrete cholecystokinin, a hormone that stimulates the gall bladder to contract and release bilebile,
bitter alkaline fluid of a yellow, brown, or green color, secreted, in man, by the liver. Bile, or gall, is composed of water, bile acids and their salts, bile pigments, cholesterol, fatty acids, and inorganic salts.
..... Click the link for more information. . There is evidence that the upper intestine secretes pancreatozymin, which enhances the amount of digestive enzymes in the pancreatic juice. In addition, the pyloric region of the stomach secretes gastrin, a hormone that increases the secretion of hydrochloric acid into the stomach. The placenta has been shown to secrete progesterone and chorionic gonadotropin. There is evidence that it even contains a substance similar to growth hormone. Insects have a unique hormonal system that includes ecdysone, a steroid that influences molting and metamorphosis, and juvenile hormone, needed for early development. Plants, too, have a hormonal system, which includes the auxinsauxin
, plant hormone that regulates the amount, type, and direction of plant growth. Auxins include both naturally occurring substances and related synthetic compounds that have similar effects. Auxins are found in all members of the plant kingdom.
..... Click the link for more information. , the gibberellins, the cytokinins, and substances associated with the formation of flowers, tubers, bulbs, and buds. Ethylene is said to function as a hormone in plants, acting to hasten the ripening of fruits.
One of the chemical messengers produced by endocrine glands, whose secretions are liberated directly into the bloodstream and transported to a distant part or parts of the body, where they exert a specific effect for the benefit of the body as a whole. The endocrine glands involved in the maintenance of normal body conditions are pituitary, thyroid, parathyroid, adrenal, pancreas, ovary, and testis. However, these organs are not the only tissues concerned in the hormonal regulation of body processes. For example, the duodenal mucosa, which is not organized as an endocrine gland, elaborates a substance called secretin which stimulates the pancreas to produce its digestive juices. The placenta is also a very important hormone-producing tissue. See separate articles on the individual glands.
The hormones obtained from extracts of the endocrine glands may be classified into four groups according to their chemical constitution: (1) phenol derivatives, such as epinephrine, norepinephrine, thyroxine, and triiodothyronine; (2) proteins, such as the anterior pituitary hormones, with the exception of adrenocorticotropic hormone (ACTH), human chorionic gonadotropin, pregnant-mare-serum gonadotropin, and thyroglobulin; (3) peptides, such as insulin, glucagon, ACTH, vasopressin, oxytocin, and secretin; and (4) steroids, such as estrogens, androgens, progesterone, and corticoids. Hormones, with a few exceptions like pituitary growth hormone and insulin, may also be classified as either tropic hormones or target-organ hormones. The former work indirectly through the organs or glands which they stimulate, whereas the latter exert a direct effect on peripheral tissues. See Endocrine system (vertebrate)